This application is based upon and claims priority of Japanese patent application1 No.11-039337 filed on Feb. 18, 1999, the contents being incorporated herein by reference.
1. Field of the invention
The present invention relates to an optical attenuation device. More particularly, the present invention relates to an optical attenuation device having at least two different type magneto-optical optical attenuation elements coupled in a cascade together.
2. Description of the Related Art
It is believed that in the beginning of the 21st century, as broadband multimedia service truly becomes widespread, the telecommunications capacity of trunk line networks will require systems in the terabit class (terabit/second), which has a hundred times greater capacity than current systems.
With this sort of demand, wavelength division multiplexing (WDM) transmission is being touted as the next generation of telecommunications technology.
WDM transmits a plurality of lights with differing wavelengths over a single optical fiber. Multiplexing along the wavelength axis allows the transmission of a large volume of data. Because the transmission rate of the wavelengths can be set low, the burden on the electronic and optical devices is relaxed, and there is little optical pulse wavelength deterioration from the non-linear characteristics of optical fiber, and the wavelength and polarized wave dispersion characteristics. This is an advantage of WDM.
FIG. 1 shows the relationship between wavelength and intensity of optical signals that have been subject to wavelength division multiplexing. In the example of FIG. 1, the n number of lights xcex1 through xcexn with differing wavelengths are disposed at intervals of 0.8 nm, and the light of each wavelength is carrying different information.
With this WDM, an optical attenuation device is prepared for each wavelength, in order to make uniform the power of the lights with varying wavelengths. Each optical attenuation device must be controlled in accordance with the power of the output light of the optical attenuation device.
Also, because the attenuation characteristic of optical fiber is somewhat different depending upon the wavelength, an optical attenuation device must apply attenuation characteristic and the reverse pre-emphasis characteristic to the pre-transmission light signal, and try to make uniform the power of the post-transmission light signal. FIG. 2 shows one example of these types of pre-emphasis characteristic. In this example, the light is emphasized in proportion to the wavelength.
Optical attenuation devices used in this manner to adjust the power of the light must appropriately adjust their attenuation characteristic in accordance with the output characteristic of the light signal. For this reason, optical attenuation devices using, for example, magneto-optical optical attenuation elements are widely used.
There are two types of magneto-optical optical attenuation elements, the D type and the I type. FIG. 3 shows the relationship between current and attenuation volume of the D type magneto-optical optical attenuation element. As shown in FIG. 3, the D type magneto-optical optical attenuation element has its peak when the current is low.
FIG. 4 shows the relationship between current and attenuation volume of the I type magneto-optical optical attenuation element. In the I type magneto-optical optical attenuation element, the attenuation volume increases in a roughly proportional manner to the current.
Conventionally, the D type magneto-optical optical attenuation element, which has a large optical attenuation volume when the current is cut, has been used. This is because unnecessary output of optical signals from a device can be prevented, even when the system is down and no control current is supplied to the element.
However, the D type magneto-optical optical attenuation element has the problem of having a characteristic curve that is more complicated than that of the I type magneto-optical optical attenuation element, thus making control difficult.
Also, the D type magneto-optical optical attenuation element has a peak in its characteristic curve, which varies depending on the temperature and is different from element to element. This creates the further problem of making accurate control difficult.
An object of the present invention is to provide an optical attenuation device that is easy to control.
Another object of the present invention is to provide an optical sending device wherein the transmission of a uniform light is possible with simple control.
Objects and advantages of the present invention are achieved by providing an optical attenuation device. The optical attenuation device comprises an optical attenuation unit including at least two different type magneto-optical optical attenuation elements coupled in a cascade together, and a current supply unit to supply control current to the optical attenuation unit.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements is a D type magneto-optical optical attenuation element and another of the different type magneto-optical optical attenuation elements is an I type magneto-optical optical attenuation element.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements comprises a faraday rotator, a polarizer and an analyzer. The polarizer has a polarization direction which is 90 degrees with respect to a polarization direction of the analyzer. Another of the different type magneto-optical optical attenuation elements comprises a faraday rotator, a polarizer and an analyzer. The polarizer has a polarization direction which is parallel with a polarization direction of the analyzer.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements is a magneto-optical optical attenuation element having a peak attenuation volume when a control current for the magneto-optical optical attenuation element is low, and another of the different type magneto-optical optical attenuation elements have an attenuation volume which increases in an approximately proportional manner to a drive current for the magneto-optical optical attenuation element.
In accordance with embodiments of the present invention, the optical attenuation device further comprises a housing housing the attenuation unit.
Objects and advantages of the present invention are achieved by providing an optical attenuation device. The optical attenuation device comprises a first magneto-optical optical attenuation element to attenuate an optical signal to output an attenuated optical signal, and a second magneto-optical optical attenuation element to attenuate the attenuated optical signal output from the first magneto-optical optical attenuation elements. The first magneto-optical optical attenuation element and the second magneto-optical optical attenuation element are different types.
Objects and advantages of the present invention are achieved by providing an optical sending device. The optical sending device comprises an optical attenuation unit including at least two different type magneto-optical optical attenuation elements coupled in a cascade together, a light being attenuated by the at least two different type magneto-optical optical attenuation elements, and a control unit to control attenuation characteristics of the optical attenuation unit to conform to a prescribed characteristic.
In accordance with embodiments of the present invention, the control unit supplies a constant current to one of the different type magneto-optical optical attenuation elements and controls the current supplied to another of the different type magneto-optical optical attenuation elements such that the attenuation characteristic conforms to a prescribed characteristic.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements is a D type magneto-optical optical attenuation element and another of the different type magneto-optical optical attenuation elements is an I type magneto-optical optical attenuation element.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements comprises a faraday rotator, a polarizer and an analyzer. The polarizer has a polarization direction which is 90 degrees with respect to a polarization direction of the analyzer. Another of the different type magneto-optical optical attenuation elements comprises a faraday rotator, a polarizer and an analyzer. The polarizer has a polarization direction which is parallel with a polarization direction of the analyzer.
In accordance with embodiments of the present invention, one of the different type magneto-optical optical attenuation elements is a magneto-optical optical attenuation element having a peak attenuation volume when a control current for the magneto-optical optical attenuation element is low and another of the different type magneto-optical optical attenuation elements have an attenuation volume which increases in an approximately proportional manner to a drive current for the magneto-optical optical attenuation element. The control unit supplies a constant current which is higher than a current whose attenuation volume has peak to one of the different type magneto-optical optical attenuation elements, and controls the current supplied to another of the different type magneto-optical optical attenuation elements such that the attenuation characteristic conforms to a prescribed characteristic.
In accordance with embodiments of the present invention, the optical sending device further comprises a housing housing the attenuation unit.
Objects and advantages of the present invention are achieved by providing an optical sending device. The optical sending device comprises a plurality of an optical output unit to output light corresponding to the input signal, a plurality of an optical attenuation unit respectively including at least two different type magneto-optical optical attenuation elements coupled in a cascade together, wherein the light output from the output unit is radiated and attenuated, and a control unit to supply a constant current to one of the different type magneto-optical optical attenuation elements first and then control the current supplied to each another of the different type magneto-optical optical attenuation elements in order such that the attenuation characteristic of the optical attenuation unit conforms to a prescribed characteristic.
Objects and advantages of the present invention are achieved by providing an optical sending device. The optical sending device comprises a decoupler to decouple a portion of an optical signal, a first attenuator to attenuate the optical signal having the portion decoupled there from to output an attenuated optical signal, a second attenuator to attenuate the attenuated optical signal output from the first attenuator, and a controller to control attenuation characteristics of at least one of the first attenuator and the second attenuator in accordance with the decoupled portion.
Objects and advantages of the present invention are achieved by providing an optical communication system. The optical communication system comprises a sending device to output an optical signal attenuated by an attenuation unit including at least two different type magneto-optical optical attenuation elements coupled in a cascade together, and a receiving device to receive the attenuated optical signal.
Objects and advantages of the present invention are achieved by providing an optical attenuation method. The optical attenuation method comprises a step of attenuating an optical signal with a first magneto-optical optical attenuation element, a step of attenuating the optical signal attenuated by the first magneto-optical optical attenuation element with a second magneto-optical optical attenuation element, wherein the first magneto-optical optical attenuation element and the second magneto-optical optical attenuation element are different types, and a step of controlling a current provided to the first magneto-optical optical attenuation element and the second magneto-optical optical attenuation element.